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The First Stars and Galaxies: Challenges for the Next Decade Participants
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Talk
Title: Three Modes of Metal-Enriched Star Formation in the Early Universe Author(s): Britton Smith Abstract: We study how the collapse of gas clouds is altered by the addition of metals by performing a series of simulations of pre-enriched star formation at various metallicities using the adaptive mesh refinement code, Enzo. In order to directly compare with the case of metal-free star formation, we use initial conditions identical to the formation of a first star, including only radiation from the high redshift cosmic microwave background (CMB). For metallicities below the classical critical metallicity, Z_cr, and in the absence of dust, collapse proceeds similarly to the metal-free case, and only massive objects form. For metallicities well above Z_cr, efficient cooling rapidly lowers the gas temperature to the temperature of the CMB. The gas is unable to radiatively cool below the CMB temperature, and becomes thermally stable. For high metallicities, log(Z/Zsun) > -2.5, this occurs early in the evolution of the gas cloud, when the density is still relatively low. The resulting cloud-cores show little or no fragmentation, and will most likely form massive stars. If the metallicity is only slightly above Z_cr, the cloud cools efficiently but does not reach the CMB temperature, and fragmentation into multiple objects occurs. We further investigate the ability of the CMB to suppress fragmentation by performing an additional simulation at log(Z/Zsun) = 2, but ignoring the influence of the CMB. This simulation shows the largest degree of fragmentation in the entire set of runs performed. We conclude that there were three distinct modes of star formation at high redshift (z > 4): a 'primordial' mode, producing massive stars (10s to 100s Msun) at very low metallicities (log(Z/Zsun) < -3.75); a 'CMB-regulated' mode, producing moderate mass (10s of Msun) stars at high metallicites (log(Z/Zsun) > -2.5) at redshift z ~ 15-20); and a 'metallicity-regulated' low-mass (a few Msun) mode existing between those two metallicities. As the universe ages and the CMB temperature decreases, the range of the low mass mode extends to higher metallicities, eventually becoming the sole mode of star formation.
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